The present invention relates to the processing of images for displaying on a display, and in particular to the processing of images for displaying images on a liquid crystal display.
Video images are displayed on various display devices such as Cathode Ray Tubes (CRTs) and Liquid Crystal Displays (LCDs). Typically such display devices are capable of displaying on a display screen images consisting of a plurality of picture elements (e.g., pixels) which are refreshed at a refresh rate generally greater than 25 Hertz. Such images may be monochromatic, multicolor, full-color, or combinations thereof.
The light of the successive frames which are displayed on the display screen of such a CRT or LCD display device are integrated by the human eye. If the number of displayed frames per second, typically referred to as the frame rate, is sufficiently high an illusion of the images being displayed in a continuous manner is created and therefore an illusion of motion may be created.
The technique in which images are formed on the display screen of a CRT display is fundamentally different from the way in which images are formed on the display screen of a LCD display. On a CRT display device the luminance of a picture element is produced by an area of a phosphor layer in the display screen where the area is struck by a writing electron beam. On a LCD display device, the luminance of a picture element is determined by the light transmittance state of one or more liquid crystal elements in the display screen of the LCD display device at the location of the picture element, whereby the light itself originates from ambient light or a light source. For accurate reproduction of moving images or moving parts of an image, the luminance response of the used display device is important.
The luminance responses and the luminance response times of CRT and LCD display screens are different. The luminance response time, being the time needed to reach the correct luminance on the display screen in response to an immediate change in a corresponding drive signal, is shorter than a frame period for a CRT display device but up to several frame periods for a typical LCD display device.
For LCD display device, the luminance responses and the luminance response times are different for a darker-to-brighter luminance transition as compared to the responses and response times for a similar brighter-to-darker luminance transition. Further, the luminance responses and luminance response times are temperature dependent, drive voltage range dependent, and, due to production tolerances, unequal over the LCD screen area (location dependent).
One existing technique to change the luminance response times with LCD display devices is to attempt to shorten the overall luminance response times by over-driving all the signals of the display for the slower of the transition of darker-to-brighter and brighter-to-darker. While of some benefit in increasing the temporal response of the display, the resulting image still includes some flickering. Flickering may be observed, in many cases, as apparent flickering of an image as the image is moved around on the display. Flickering tends to be most pronounced when an image is viewed on a shaded background with a dotted pattern as well as vector art often used in computer aided drawings.
Another existing technique to change the luminance response times with LCD display devices is to slow down the transition of all pixels of the display from the darker-to-brighter transition and the brighter-to-darker transition to the slowest transition within the display. This slowing down of the transition may be performed by modification of the driver waveform to achieve the slower temporal response. While slowing down the transition of all the pixels of the display results in a decrease in apparent flicker, unfortunately, the slowing down of the temporal response of the entire display result in objectionable motion blur because of the insufficient effective refresh rate.
EP 0 951 007 B1 disclose a de-flickering technique in which the video signal is modified so that the asymmetry of luminance rise and decay time is compensated. EP0 951 007 B1 is incorporated by reference herein. Referring to FIG. 1, FR which is representative of the present luminance output as it was predicted one frame before (previous frame) is subtracted from the input video signal. This difference and the present luminance output FR are the two inputs to the processing unit. The outputs of the processing unit are ΔC and ΔR, where ΔC is the new correction value to be added to the present predicted luminance FR, and ΔR is the new prediction of luminance change after the next frame.
K. Sekiya and H. Nakamura (IBM), in a paper entitled “Overdrive Method for TN-mode LCDs—Recursive System with Capacitance Prediction,” SID'01, pp114-117; H. Nakamura and K. Sekiya (IBM), in a paper entitled “Overdrive Method for Reducing Response Times of Liquid Crystal Displays,” SID'01, pp. 1256-1259; and H. Nakamura, J. Crain, and K. Sekiya (IBM), in a paper entitled “Computational Optimization of Active-Matrix Drives for Liquid Crystal Displays,” IDW'00, pp.81-84; address some fundamental issues in overdrive technologies. These papers collectively suggest that while reducing the temporal response time of LCDs is the single goal in many overdrive technologies, the generally accepted definition of temporal response time is inappropriate,tresponse=tarrival−tstart where tarrival is the time of the arrival point. The arrival point is defined asvarrival=vstart+90%×(vtarget−vstart)where vstart is a starting brightness value and vtarget is a target brightness value. By this definition, the arrival point of the same target values varies by different starting values. Accordingly these papers suggest that if the difference between a starting and a target values is large, the arrival point is too offset from the target value.
These papers further suggest that the current overdrive technologies are ineffective because the overdrive technologies make the assumption that LC molecules in pixels always successfully transit from an equilibrium state to another equilibrium state within a driving cycle, and consequently ignore the fact that although an overdrive value is only applied to a pixel for one driving cycle, the overshot effect on that pixel lasts for several driving cycles. The current overdrive technologies typically store the brightness of a frame, and use a brightness-based lookup table.
To reduce these two problems, the papers proposed a new definition of temporal response time by re-defining the arrival point as a constant tolerance from a target value (gamma correction is considered), and a recursive overdrive scheme that stores internal capacitance of a frame. The papers suggest that the internal capacitance of a pixel plays a critical role in determining the brightness of the pixel, and therefore, internal capacitance of every pixel, but not the brightness of every pixel, should be stored. Because internal capacitance can not be obtained directly, it is estimated. Specifically, the estimation of a pixel's internal capacitance at time n is based on the previous estimation at time n−1 and the driving value at time n, resulting in a recursive implementation structure.
The papers further suggest phenomena in LCDs driven by most existing brightness-based overdrive technologies is that although an overdrive value applied to a pixel in one driving cycle makes the pixel reach a desired target value, if the normal driving value that is associated with that desired target value is applied to that pixel in the following driving cycles, the pixel surprisingly cannot sustain the target value that it achieved in the overdriving cycle, resulting in overshooting/undershooting effects in the following driving cycles. Brightness-based non-recursive overdrive schemes cannot solve this problem because they assume that an actual display value of a pixel can reach a target value and the LC molecules of that pixel reach an equilibrium state in an overdriving cycle, and this assumption is not true in reality. Although a pixel may achieve the desired target value in an overdriving cycle, the LC molecules of that pixel have not reached the corresponding equilibrium state.
According to the papers, the internal capacitance-based recursive overdrive scheme overcomes this problem. The scheme more precisely describes the intrinsic properties of TFT LCD by tracking the internal capacitance change, so it can better deal with the overshooting/undershooting effects in the brightness-based non-recursive overdrive schemes as follows:                Overshooting effect after an overdriving cycle: Upon the value of the estimated internal capacitance after an overdriving cycle, the new scheme has the capability of applying another overdrive in the reverse direction in the next driving cycle.        Undershooting effect after an overdriving cycle: Upon the value of the estimated internal capacitance after an overdriving cycle, the new scheme has the capability of applying another overdrive in the same direction in the next driving cycle.        
As it may be observed, these papers use one-frame overdrive technologies based upon a model that assumes that transitions are always finished within a driving cycles, starting from an equilibrium state and finally ending at an equilibrium state. The recursive nature of the technique is internal to a single frame.
Furthermore, these papers are premised on the following theory. Charge (O) is injected into the display during a short time interval by applying a voltage and then the charge is held in the display by open-circuiting the charge source. Accordingly, the amount of charge Q is fixed during a frame. However, the applied voltage is changed to zero during the rest of the frame upon open-circuiting the source. Thus the capacitance of the pixel changes during the rest of the frame, namely Capacitance=Qinjected (which is fixed)/Vapplied (which is changing toward zero). The voltage maintained across the pixel changes with the changing capacitance, the luminance output then varies as related to the injected charge, which is dependent on the particular drive scheme being used to inject the charge. Accordingly, the capacitance model proposed above does not have an inherent 1 to 1 mapping between capacitance parameters and luminance values (e.g., a capacitance value can be related to multiple luminance values), which makes determining the appropriate values problematic.
K. Kawabe, T. Furuhashi, and Y. Tanaka (Hitachi), in a paper entitled “New TFT-LCD Driving Method for Improved Moving Picture Quality,” SID'01, pp 998-1001, suggest that the existing ways to determine overdrive values, as to make actual display values and desired values as close as possible, cannot fully eliminate motion blur, because it fails to consider the effect of long transitions before reaching the desired values. In order to compensate for visual effects of long transitions, Kawabe et al. propose a dynamic contrast compensation (DCC) method with stronger overdrive values that make actual display values surpass the desired values, as illustrated in FIG. 2. In effect, to compensate for the inability of the display to achieve the desired values they suggest using a modified overdrive waveform.
Rho, Yang, Lee, and Kim (Korea), in a paper entitled “A New Driving Method For Faster Response of TFT LCD on the Basis of Equilibrium Charge Injection,” IDW '00, pp. 1155-1156, suggest a theoretical description of the overdrive voltage as:
      V    overdrive    =            (                                    C                          LC              -              target                                +                      C            s                                                C                          LC              -              current                                +                      C            s                              )        ⁢          V      target      where CLC-target is the equilibrium capacitance of the next frame, CLC-current is the current capacitance, Cs is the storage capacitance, and Vtarget is the target voltage. If correct, this representation quantifies in some manner the value in using pixel capacitance.
Okumura, Baba, Taira, Kinno (Korea) in a paper entitled, “Advanced Level Adaptive Overdrive (ALAO) Method Applicable to Full HD-LCTVs,” SID '02, model the LCD by a one-tap IIR filter. Then overdriving circuitry, as the inverse of the LCD response, is a one-tap FIR filter. Okumura, et al. suggest using the signal-to-noise (S/N) ratio by not applying the overdrive if the S/N of an input frame is too low (below a certain threshold). Okumura, et al. also propose the concept of dynamic resolution as an evaluation measure replacing widely used “temporal response time.” It is noted that in K. Sekiya and H. Nakamura (IBM), in the paper entitled “Overdrive Method for TN-mode LCDs—Recursive System with Capacitance Prediction,” SID'01, pp114-117, discussed above, address the same issue and propose a different solution, namely, re-defining the “temporal response time.”
B-W Lee et. al., in a paper entitled “Reducing Gray-Level Response to One Frame: Dynamic Capacitance Compensation,” SID '01, and B-W Lee et al., “LCDs: How fast is enough?” SID '01, pp1106-1109, subjectively tested motion artifacts and showed that: (1) even 0-response time LCDs can still have certain motion blur due to the hold-type display scheme; (2) when the response time of LCDs is reduced by less than half a frame, the quality of moving objects is almost as good as that of 0-response time LCDs. Specifically, “since the fastest change in today's video sources is 1/30 sec, the LCD's response needs to be within 1/60 sec.”; (3) variation of operational temperature of LCD cells affects overdrive. “Since switching speed and dynamic capacitance change as a function of temperature, a set of compensation values measured at a certain temperature will yield different results at other temperatures.”; and (4) over-compensated overshoot is similar to the edge enhancement technique. Accordingly, inaccurate overdrive voltages are not terribly accurate, due to many factors, such as the temperature.
JP 64-10299 disclose a LCD control circuit that compares the input data with the data written in the frame memory from the previous frame. Only in the event that the input data is larger than the stored data is corrective data determined. The corrective data is applied to the LCD control circuit to provide overdrive. JP 64-10299 specifically teach that in the event that the input data is smaller than the stored data, then the corrective data is not determined, but rather, the input data is provided directly to the LCD control circuit. The corrective data or the input data, depending on the comparison is provided to the frame memory. The JP 64-10299 reference tends to exhibit uneven edges in the image, a higher than expected contrast in different regions of the display, a lower than expected contrast in other regions of the display, a higher than expected increase in sharpness in some regions of the display, a lower than expected decrease in sharpness in other regions of the display, and a blurring of other portions of the display.